YOU GOT A "SAGGY BOX"?
Lol Jon.....Would you believe I was just working on that "saggy box" affliction when I got your post
OK Fans! Here It Is!!
Because of the way the underframes on Athearn cars are injection molded they can be distorted and really don't fit right to obtain the proper coupler height.
Install an underframe and trucks on an Athearn car. Now take a look at the coupler boxes. Notice that there is a small gap between the top side of the coupler box and the bottom side of the carbody. When the truck screws are tightend the gap will become even more pronounced.
Here's the fix.....Install the underframe and trucks....be sure that your trucks are screwed down tight...we don't want them to turn right and left yet.
Now check the ends of the car to see if you have any gap...If you see a gap proceed to the next step...If you don't see a gap loosen the truck screws until the gap appears.
Using a straight pin and super glue place a small drop of the glue on top of the coupler box and hold it against the carbody until it sets. Do the same on the other end.
Now install the couplers and either tighten or loosen your truck screws until the trucks rotate right and left freely. You should now have a coupler height that is within NMRA tolerences without having to shim the trucks with washers.
Added bonus....Combined with my post about "nubbins" you now have couplers that stick "straight out" and no "saggy box"